The use of vacuum hot pressed intermetallic composites as self resistance heated evaporation sources is well established. These composites are employed in the evaporation of aluminum and wide range of other metals and alloys. One class of these composites comprises binary intermetallics composed of titanium diboride (TiB.sub.2) and boron nitride (BN). Another class of these composites comprises ternary intermetallics including the above-named materials with the addition of aluminum nitride (AlN). These composites are available in a wide range of resistivities of from about 50 to 2000 .mu..OMEGA.-cm (microhm centimeter). As evaporation sources, e.g. for aluminum, these composites are generally considered to have served their useful life when their resistivity has fallen to a value of about 50% of their initial value. At this point, the amperage required to maintain a constant evaporation rate is deemed to be excessive from an economic standpoint.
It has been postulated that the failure is caused by vaporization of some of the composite materials during use. The vaporization leaves voids in the composite, which voids are replaced by the molten evaporant, e.g., aluminum, thus lowering the resistivity of the composite.
When aluminum is the evaporant, another failure mechanism that can occur is the formation of an anti-wetting surface due to the reaction of the molten aluminum with oxide impurities in the composite. It is known that commercial grades of titanium diboride and boron nitride contain small but significant amounts of boron oxide as B.sub.2 O.sub.3. The reaction has been suggested to occur as follows: EQU 10 Al+3 B.sub.2 O.sub.3 +3 TiB.sub.2 .fwdarw.6 AlB.sub.2 +3 TiO+2 Al.sub.2 O.sub.3. (1)
The AlB.sub.2 represents crystal accumulation while the TiO and Al.sub.2 O.sub.3 provide an anti-wetting surface.
When aluminum is the evaporant a reaction between it and boron nitride in the composite is less likely because of the low contact affinity between them; but, again, in the presence of oxides, such as for example B.sub.2 O.sub.3, a reaction can occur which will produce compounds which restrict the evaporation surface, this reaction proceeding as follows: EQU 3 Al+B.sub.2 O.sub.3 .fwdarw.Al.sub.2 O.sub.3 +AlB.sub.2. (2)